The present invention relates generally to the field of screen printing. More particularly, the invention provides a method for printing a graphic having uniform ink density on a printing screen coated with a light-sensitive emulsion.
Pending U.S. patent application Ser. No. 628,620, assigned to the present assignee, discloses a method and apparatus for printing a graphic on a light-sensitive emulsion which has previously been applied to one surface of a printing screen. As disclosed therein, an ink-jet or thermal printer may be used to print the graphic either directly on the emulsion or on an ink receptive coating applied to the emulsion. It has been found that in some instances the ink density of the graphic is non-uniform. This is particularly the case where the graphic includes extensive areas of solid ink.
The problem results from the fact that the surface tension of the ink prevents it from being immediately and uniformly absorbed into the emulsion as the ink is applied. Instead, the ink tends to be drawn to the center of the printed area. This effect is illustrated in FIGS. 1(a) and 1(b). FIG. 1(a) shows an area of solid print 10 immediately after it has been applied to a light-sensitive emulsion 12 by, for example, the single pass of an ink-jet printhead across the emulsion. FIG. 1(b) shows the same printed area a short time after the ink has been applied. As illustrated in FIG. 1(b), the ink migrates to the center of the printed area to form a band 14 of relatively high ink density and two bands 16, 18 of relatively low ink density.
As illustrated in FIG. 2, the ink from the next pass of the printhead across the emulsion produces the same effect. Thus, as successive lines of print 20, 22, 24 are applied to the emulsion 12, areas 26, 28, 30 of high ink density alternate with areas 32, 34, 36, 38 of relatively low ink density. These areas or bands of low ink density are referred to as scan lines, and the low ink density of the scan lines is insufficient to block UV light when the light-sensitive emulsion is exposed. Thus, after the emulsion is washed out, a scan line of unexposed emulsion is left embedded in the finished screen, rendering the screen unsuitable for printing purposes.
Accordingly, it is an object of the present invention to provide a method for printing a graphic on an emulsion coated printing screen wherein the problem of scan lines is eliminated.